1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically to a semiconductor device with a transistor cell with a polysilicon gate electrode and an interconnect line containing aluminum.
2. Description of the Background Art
Semiconductor elements (MOSFETs (metal oxide semiconductor field effect transistors), IGBTs (insulated gate bipolar transistors) and others) formed by using silicon carbide (SiC) are promising candidates for next-generation switching elements capable of achieving high breakdown voltage, low loss, and high resistance to heat, and are expected to be applied to power semiconductor devices such as inverters.
A generally employed structure of a conventional MOSFET using SiC (SiC-MOSFET) is such that a silicide layer to make ohmic contact is formed on a source region, and a source electrode made of aluminum (Al) is formed on the silicide layer (see for example Japanese Patent Application Laid-Open No. 2009-194127). In the structure disclosed in Japanese Patent Application Laid-Open No. 2009-194127, a metal layer made of Ti is interposed between a silicide layer on a source region and an aluminum source electrode, and the metal layer functions as a barrier metal to suppress diffusion of Al.
A reliability test such as an HTGB (high temperature gate bias) test conducted by continuously applying a voltage between the gate and the source shows that the conventional SiC-MOSFET suffers from reduction with time in a gate-to-source threshold voltage (VGSth).
Reduction in the threshold voltage increases the transfer characteristics (ratio of output to input) of the MOSFET to generate flow of overcurrent during the actual use of the MOSFET, resulting in a fear of breakage of the MOSFET. Reduction in the threshold voltage also increases a switching speed during turn on. This generates nonuniformity of the operations of a plurality of MOSFET cells of a semiconductor chip, resulting in a fear of breakage of the semiconductor chip. Further, even if no problem in electric characteristics is found as a result of the test, the threshold voltage may be reduced due to prolonged application of voltage stress between the gate and the source, so the aforementioned problem is likely to occur.
SiC devices achieve excellent electric characteristics at high temperatures, so they are expected to be used in a high-temperature condition. Meanwhile, Al forming a source electrode may cause corrosion of an interlayer insulating film intended to maintain the isolation between the gate and the source, or may diffuse into polysilicon forming a gate interconnect line that is what is called “Al spike,” resulting in a fear of formation of a short circuit between the gate and the source.